Update: WordPress 4.2 has full UTF-8 support! There’s no need to upgrade manually any more.
For many years, MySQL had only supported a small part of UTF-8, a section commonly referred to as plane 0, the “Basic Multilingual Plane”, or the BMP. The UTF-8 spec is divided into “planes“, and plane 0 contains the most commonly used characters. For a long time, this was reasonably sufficient for MySQL’s purposes, and WordPress made do with this limitation.
It has always been possible to store all UTF-8 characters in
latin1 character set, …